The encapsulating of electronic devices which include integrated circuit die, mounted on the surface of a substrate and wire bonded to achieve electrical interconnection to the substrate, can be accomplished by transfer molding or flow forming.
Transfer molding involves the use of a thermoset plastic, typically novolac modified epoxy resins, injected at high temperatures and under high pressures into a metal die cavity covering the top of the substrate to which is attached one or more bonded die, typically in an array that is later separated into individual devices. In this process a backing plate for the die covers the bottom of the substrate to prevent the material from being pressed out the bottom through the many plated through holes that typically exist in the substrate for electrical interconnection between the top and bottom of the substrate.
Flow forming, also known to one of ordinary skill in the art as glob top packaging, involves the use of a different thermoset plastic, such as epichlorhydrin Bisphenol-A epoxy, which is formed at lower temperatures, and relies upon a low viscosity of the material at the forming temperature, a dam around the periphery of the substrate to constrain the lateral flow, and gravity to form the protective cover for the device. Flow forming has advantages over transfer molding including the ability to typically handle larger arrays, and simpler processing machinery, making it a lower cost process. In flow forming, the adhesion of the cover to the substrate has relied on adhesive properties that occur between the flow material, typically an epoxy encapsulating material, and the surface of the substrate, typically ceramic or glass epoxies or mask or film surfaces on the surface of the substrate. The flow forming method has been used successfully in the past because the area of the attachment surface is large compared to the total covered area and the substrate is substantially inflexible within the covered area. Because products are growing smaller to meet market demands and integrated circuit die are growing larger with more and more outputs, the surface area covered by the flow formed material compared to the total substrate area is becoming smaller. Furthermore, the need to make complex devices more economically is leading to a choice of inherently less expensive, more flexible substrate materials such as glass epoxy instead of ceramic.
Under these circumstances, the reliability of the attachment of the flow formed cover to the substrate can become unacceptable, with the top loosening from the substrate, or from the film on the substrate, during electrical testing, as in that done with a spring pin test fixture having many pins, or under the stresses of punching an integrated circuit device encapsulated by a flow formed cover out of an array of such devices.
A similar problem exists in devices encapsulated by transfer molding. An improvement usable with transfer molding is taught in U.S. Pat. No. 5,136,399, issued on Aug. 4, 1992 to Worp et al. In this technology, the shape of the bottom end of the cover's attachment anchor pin is determined by the bottom mold cavity, or plate. This invention has shown merit for devices fabricated using transfer molding techniques but would complicate and increase the cost of flow formed molding by requiring a similar mold under the substrate during the cover forming process. Thus, what is needed is a means to improve the attachment of the flow formed cover to the substrate in a flow forming process.